win德赢

Abstract: Oxygen isotope (δ18O) of paleolake water is a key indicator for reconstructing the formation temperature and diagenetic history of lacustrine carbonate minerals. In this study, we use clumped isotopes (Δ47) of lacustrine shell limestones to determine the surface erosion thickness and δ18O of Early Jurassic lake water in the northern Sichuan Basin. We analyzed nine shell limestone and seventeen shale samples from the Early Jurassic Da'anzhai Member (J1z4) in the Yuanba area. Whether the shell's shapes are well or partially preserved, the J1z4 shell limestones in the Yuanba area show no significant recrystallization and dull cathodoluminescence. These characteristics suggest that the shell limestones did not undergo significant diagenetic alteration during late burial. The Δ47 values of the J1z4 shell limestones range from 0.448 ± 0.005‰ to 0.463 ± 0.006‰, yielding clumped isotope temperature (TΔ47) of 64.4 ± 0.8 to 69.7 ± 1.4 °C, which is significantly higher than the Early Jurassic paleotemperature. It suggests that the Δ47 of shell limestones was altered by solid-state reordering, meaning the TΔ47 does not reflect the initial formation temperatures. By integrating organic matter's maturation model (Easy%Ro) of coexisting shales with Δ47 solid-state reordering model of calcite, we constrained the maximum burial temperature (～170 °C) and the initial formation temperature (～28 °C) of these shell limestones. Based on reported paleotemperature gradient, we estimated that the surface erosion thickness was about 1500 m. Furthermore, using the determined initial formation temperatures and conventional oxygen isotope thermometer, we determined that the δ18O values of the Early Jurassic lake water in the Sichuan Basin, which ranged from ?10.8‰ to ?8.0‰ (SMOW). The reconstructed paleotemperature and δ18O of lake water suggest that the Early Jurassic in the Sichuan Basin was warm and humid, which was favorable for the deposition of organic-rich lacustrine shale. The methods developed in this study, which employ Δ47 of shell limestones to reconstruct the maximum burial temperature and paleolake environmental conditions, demonstrate broad applicability to the Sichuan Basin and similar lacustrine basins.